Release Of Stored Heat Energy To Do Useful Work

ABSTRACT

This invention concerns the release of stored heat energy to do useful work. In particular it concerns a method for the release of stored heat energy to do useful work, and in further aspects it concerns other methods, apparatus and systems. In a first aspect, the invention is a method for releasing stored heat energy to do useful work. The method comprising: Storing heat energy in a heat storage medium. Arranging a surface of the heat storage medium and a surface of a heat collecting element of a heat consuming engine or process, such that a surface of the heat collecting element is in proximity to, in contact with or embedded in a surface of the heat storage medium. Then, transferring stored heat energy from the surface of the heat storage medium to the surface of the heat collecting element.

TECHNICAL FIELD

This invention concerns the release of stored heat energy to do useful work. In particular it concerns a method for the release of stored heat energy to do useful work, and in further aspects it concerns other methods, apparatus and systems.

BACKGROUND ART

By ‘renewable energy’ we mean energy from any source, including wind, solar, geothermal, ocean energy and bioenergy sources, that is naturally replenished. Renewable sources of energy tend to be variable and intermittent in their output. For instance, solar energy collected by photovoltaic cells generates electricity at constant voltage during sunlight hours, but varies in current according to the brightness of the sun at any particular instant. At night no electricity is generated. Accordingly the generation of electricity from such sources is used in limited ways, such as for domestic hot water systems, or to top-up mains supply. In remote and hot areas solar energy is often used in conjunction with diesel generators.

Energy storage devices can help to smooth both the short term and long term fluctuations in renewable energy. Despite the continual development of new charge storage technologies, the storage of electrical energy is still currently expensive. This has lead researchers, including the applicant, to develop storage facilities for heat energy. Carbon, and in particular a solid graphite body, has been found to be a suitable medium in which to store heat energy. However, there are many other suitable media including concrete and molten salt.

One of the challenges that arises from storing heat energy, whether it is renewable energy or not, is releasing the heat energy in a regulated fashion and delivering it, without excessive losses, to an engine able to use the heat energy to do useful work. In conventional large scale energy generating installations, steam has been used as a heat transfer medium. For instance, to transfer heat energy from a furnace to a turbine that generates electricity. However, in smaller, scale installations using renewable energy, such arrangements tend to operate at uneconomical efficiencies.

DISCLOSURE OF THE INVENTION

In a first aspect, the invention is a method for releasing stored heat energy to do useful work. The method comprising:

-   -   Storing heat energy in a heat storage medium.     -   Arranging a surface of the heat storage medium and a surface of         a heat collecting element of a heat consuming engine or process,         such that a surface of the heat collecting element is in         proximity to, in contact with or embedded in a surface of the         heat storage medium.     -   Then, transferring stored heat energy from the surface of the         heat storage medium to the surface of the heat collecting         element.

Since the step of storing heat energy is decoupled from the step of transferring the stored heat energy, the heat energy received by the engine or process need not be subject to any fluctuations in energy collection. This makes the method suitable for use where energy is collected from variable or intermittent sources, such as renewable sources. Furthermore, the step of storing heat energy may receive heat energy from more than one source of energy; for instance from several separate energy collectors.

A collector may be provided to collect or concentrate energy from the source. A renewable energy collector may be some form of solar collector, such as a reflective dish or mirror array which will redirect and focus the sun's rays into the heat storage facility. Particular examples include parabolic dish reflectors, linear parabolic trough reflectors and linear Fresnel reflector systems.

The heat storage medium may comprise a quantity of natural or synthetic graphite. The medium may be encased in a jacket of thermal insulation. Various other heat storage media could also be used, such as concrete or molten salt. The heat storage medium could be mounted at the top of a tower, say twenty-five meters above the ground or at or near the focal point of a dish collector. Where the heat storage medium and heat consuming engine are coupled together, they may both be mounted at the top of a tower or at the focal point of a dish collector. Alternatively, the tower may support a secondary reflector which reflects the concentrated solar energy back down to a graphite storage facility on or below the ground.

In any event a heliostat may be associated with the heat collector to ensure it can maintain a defined focal point during the course of the day.

The graphite block may comprise a cavity to receive concentrated solar energy, and there may be an aperture in the surface of the block, and in any thermal insulation jacket, to allow the solar energy to pass into the cavity. In general the aperture will allow, for instance, for collection of reflected sunlight from a large array of mirrors.

The graphite block may weigh ten tonnes, more or less, in weight. In use the block of graphite may reach temperatures up to ˜2000° C. while storing heat.

The graphite block may be a single monolith, or it may comprise plural smaller graphite bricks assembled together. Passageways may be drilled into or through the monolith, or the bricks of graphite, to provide channels for a heat transfer medium. The heat transfer medium could be solid or fluid. Where the heat transfer medium is fluid it may circulate in the passageways.

Where the graphite block is made up of plural bricks, the bricks may have passageways extending through them. Alternatively the bricks may have trenches along their surface. The trenches may remain open, or be closed by the surface of adjacent bricks to form passageways.

Piping may be laid into the trenches or passageways to contain the heat transfer medium. This is especially useful where the heat transfer medium is not confined entirely within the heat storage medium. For instance, the piping may convey the heat transfer medium around a process loop.

In any event the heat transfer medium could be used for heating the heat storage medium, redistributing heat around the heat storage medium or for extracting heat from the heat storage medium. Since the heat storage medium may remain at high temperatures for sustained periods of time, when piping is used it may be fabricated using materials suitable for high temperate operation, for instance ceramics or steel.

Transferring heat energy from the heat storage medium generally involves heat passing out through a surface of the medium. It also generally involves heat passing into a surface of the heat collecting element of the heat consuming engine or process. There could, of course be more than one heat collecting element connected to the heat storage medium.

In the simplest case the surface of the heat storage medium through which heat passes is a flat part of the outer surface of the medium. Similarly, the surface of the heat collecting element through which heat passes may be a flat part of the outer surface of the element. The two flat parts, of the medium and element, may be the same size and they are arranged in contact with each other to form a junction. Of course the flat part, in either case, may be recessed, or proud, relative to the surface around it.

In this simple case the heat flow from the surface of the heat storage medium is dependant on a number of different factors. For instance, the amount of heat input to the medium, the thermal mass of the medium, the thermal flow characteristics of the medium, the shape of the medium or the insulation jacket around the medium.

Alternatively, or in addition, the rate of heat flow from the surface of the heat storage medium into the heat collecting element may be controlled by altering the position, size or shape of the surface of the medium or element, or both, through which heat passes. Of course the relative locations, sizes or shapes of the two surfaces, to each other will also have an effect on heat flow rate into the element.

The heat storage medium may be equipped with heat transfer pathways. These pathways may be internal, external, or both. Their purpose is to change the thermal gradients within the medium to regulate the flow of heat out of the surface and into the heat collecting element. This may be assisted, or even achieved, by use of the orientation of the material of the medium; for instance by arranging the grain of graphite material within the heat storage medium.

Alternatively, heat transfer pathways may involve metal rods attached to the surface, or inserted through the heat storage medium. Alternatively, or in addition, they may involve a fluid circuit that moves heat from one part of the storage medium to another; for instance from the interior to an outer surface.

Of course the surface of the heat storage medium through which heat passes need not be flat. Similarly, the surface of the heat collecting element through which heat passes need not be flat. The surfaces could be dished or shaped in any useful way. They may match each other, or not.

There may be a control interface interposed between the heat collecting element and the heat storage medium. The control interface will sit between the surfaces of the medium and element through which heat flows, however it need not cover both surfaces entirely or have the same shape in contact with both surfaces.

It will be appreciated that where there is an insulating jacket around the heat storage medium, the control interface may be associated with that jacket. For instance, it may be integrated into the jacket or located inside the jacket.

The control interface may operate to control the flow of heat between the surfaces of the medium and element through which heat passes. The control interface could be made of highly conductive material such as copper. The control interface may be passive or active.

A passive control interface may have a thermal design that dissipates excessive heat when it is above a given temperature. This heat may be dissipated back into the heat storage medium. Alternatively, the control interface may recover heat from other parts of the heat storage medium when the temperature falls. The control interface may, of course, both dissipate and recover heat, as the situation demands.

An active control interface may effect control by reconfiguring itself, or by actuating moving parts. For instance, the control interface may adjust the size, shape or location of the area it has in contact with the surface of the storage medium, or element, or both. One particularly useful way of effecting control may be changing the distance between the surfaces of the heat collecting element and the heat storage medium through which heat passes. The configuration may be changed slowly as conditions change, or it could be cycled with a variable duty cycle to effect control.

An intermittent separation might be accomplished making use of the differential thermal expansion provided by different materials (like the bi-metallic strip). A mechanism could be constructed using this principle and arranged to respond to the temperature at the heat collecting element to move the heat consuming engine relative to the block. A change in the distance between them may be used to regulate the heat transfer and keep the temperature of the element within its preferred operational range.

The mechanism may be applied around the hot port of a heat consuming engine, or between the hot port and the block. An air gap may be created when the mechanism responds to excess heat, providing a binary step function control. Alternatively, some material may be used to fill the space between the engine and block, for instance a fluid filled bladder or piston may occupy the space and move as the gap changes to provide smoother control.

Another active control interface could involve the use of a heat transfer medium between the surfaces of the heat storage medium and the heat collecting element through which heat passes. In this case there could be a cavity at the point of contact which is flooded with heat exchange medium. Then, heat transfer may be regulated, for instance by controlling the pressure of the heat transfer medium. Other possibilities include controlling the flow rate of the heat transfer medium through a heat exchange circuit.

The heat consuming engine or process could be any thermodynamic engine or process including turbines, Brayton or Rankine cycle machines or process.

In one application the heat consuming engine may be a heat engine, for instance a Stirling engine. A surface of the heat collecting element of the engine (that is the hot port of a Stirling engine) may be brought into contact with a surface of the heat storage medium. For example, it may be placed directly on it, or attached directly to it; for instance so that it rests directly onto the surface of a graphite block. Alternatively a control interface may be used.

In the case of currently available Stirling engines, there are minimum and maximum operating temperatures for the heat collecting element. This predicates temperature as a control input to the heat transfer process.

Where the heat storage medium and the element of a Stirling engine are permanently fixed together, controlling the flow of heat energy from the surface of the heat storage medium may focus on control of the heat input.

The heat energy that flows through from the heat storage medium may be converted by the engine to do any kind of useful work. For instance a Stirling engine may be used to convert the heat energy into electricity.

In another application the heat consuming engine may be a supercritical carbon dioxide turbine system.

In a second aspect, the invention is an apparatus for releasing stored heat energy to do useful work, comprising:

-   -   A heat storage medium having a heat transfer surface from which         heat is transferred from the heat storage medium.     -   A heat consuming engine or process having a heat collecting         element which also has a heat transfer surface to receive heat.     -   Wherein, the heat transfer surface of the heat storage medium         and the heat transfer surface of the heat collecting element are         arranged such that the surface of the heat collecting element is         in proximity to, in contact with or embedded in the surface of         the heat storage medium to allow transfer of stored heat energy         from the heat storage medium to the heat collecting element.

In a third aspect the invention is a system for generating electricity from a variable source of energy, comprising:

-   -   A variable energy source.     -   A heat storage facility to receive the energy from the source         and store it as heat.     -   A closed loop heat transfer circuit containing a recirculating         heat transfer medium, having a heat receiving element extending         through the heat storage body and a heat delivery element to         supply heat energy.     -   And a regulator for the closed loop heat transfer circuit to         govern the temperature of the supplied heat independently of the         temperature of the heat storage facility.

There are many suitable heat transfer media available for use in the heat transfer circuit. Working fluids such as a biphase water/steam combination may be employed as a heat transfer medium. However, a working fluid having a single phase is preferred. A suitable liquid phase working fluid may be an oil. A suitable gaseous phase working fluid may be a gas that is inert with respect to the heat transfer circuit materials, and carbon dioxide (CO₂) is currently preferred.

In an alternative, simple convection of air, or any other suitable fluid, around a closed loop heat transfer circuit may suffice. In this case the surface of the heat storage facility may provide the heat receiving element, and another surface at the point of delivery may provide the heat delivery element.

The regulator for the heat transfer circuit may regulate the flow of the heat transfer medium, in particular to control the temperature of the medium entering the heat delivery element. The temperature of a graphite block heat storage facility may reach temperatures of ˜2,000° C., and may vary in temperature significantly; particularly in this case between day and night when the sun does not shine. However, it is preferred to keep constant the temperature of the medium entering the heat delivery element.

The regulator for the closed heat transfer circuit may regulate the pressure of the working gas within the heat transfer circuit in order to control the temperature of the medium entering the heat delivery element. This simple technique avoids the complications of attempting to match the quantity of heat energy being delivered to the energy requirements of the downstream engine or process. Using a single (gaseous) phase working gas simplifies pressure regulation.

The heat energy supplied by the heat delivery element may be used for a range of purposes.

A Stirling engine may be arranged to receive heat at its hot port and to drive an electricity generator. Stirling engines convert temperature difference to rotary motion. They have a cold port and a hot port. In this case the cold port may simply have heat radiating fins. However, the hot port may have an external heat exchanger element extending into a sealed chamber that is in circuit with the heat transfer circuit. The sealed chamber may for instance, be a dome built onto the Stirling engine's casing over the hot port.

At the Stirling engine the working gas is delivered to flood the sealed chamber at the hot port of the Stirling engine. This chamber may contain an external heat exchanger element of the Stirling engine, and this element will be completely immersed in the delivered working gas. The working gas may be regulated to maintain the temperature of the sealed chamber fixed at any desired temperature; for instant at 720° C. which has been found to be the current upper temperature limit of the Stirling engine hot part components due to material limitations.

In an alternative arrangement the external heat exchanger element of the hot port of the Stirling engine may be incorporated into the heat receiving element of the system of the invention.

The rotary motion output of the Stirling engine may then be used to generate electricity, for instance by turning a generator.

In a fourth aspect the invention is a system for generating electricity from a variable source of energy, comprising:

-   -   A variable source of energy.     -   A heat storage facility to receive energy from the source and         store it as heat.     -   A heat collecting element of a heat consuming engine or process,         that is in proximity, contact or embedded in the heat storage         body to supply heat energy to the heat consuming engine or         process.

In particular the heat collecting element may be an external heat exchanger element at the hot port of a Stirling engine that collects heat from the heat storage facility by conduction.

In a fifth aspect the invention is a system for generating electricity from a variable source of energy, comprising:

-   -   A variable source of energy.     -   A heat storage facility to receive the energy and store it as         heat.     -   A Stirling engine to receive heat at a hot port and to drive an         electricity generator.     -   A heat transfer circuit containing a heat transfer medium, and         having a heat receiving part extending through the heat storage         body and a heat delivering part at the hot port of the Stirling         engine.

Micro-Turbine

A micro-turbine may substitute for the Stirling engine. In this case the heat energy supplied by the heat delivery element may be used to heat a separate working fluid, for instance water/steam, for the micro-turbine.

Other Uses

The heat energy supplied by the heat delivery element may be used for a variety of different purposes. For instance, the heat could be used to pre-heat feed water, or to super heat the working gas, used in a conventional coal or gas fired power station. Alternatively, the heat could be used for process steam generation, at refineries and at chemical plants.

In a sixth aspect, the invention is a method for generating electricity from a variable source of energy, comprising the steps of:

-   -   Collecting energy from a source of renewable or variable or         intermittent energy.     -   Storing collected energy in the form of heat energy.     -   Transferring heat from the storage facility via a closed loop         heat transfer circuit containing a recirculating heat transfer         medium.     -   And regulating the closed loop heat transfer circuit to govern         the temperature of the supplied heat independently of the         temperature of the heat storage facility.

In a seventh aspect, the invention is a method for generating electricity from a variable source of energy, comprising the steps of:

-   -   Collecting energy from a source of renewable or variable or         intermittent energy.     -   Storing collected energy in the form of heat energy.     -   Transferring heat to a heat collecting element of a heat         consuming engine or process, that is in proximity, contact or         embedded in the heat storage body to supply heat energy to the         heat consuming engine or process.

In particular the heat collecting element may be an external heat exchanger element at the hot port of a Stirling engine, and the step of transferring heat may involve conduction or radiation, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a pictorial diagram of a solar collector for use with the invention.

FIG. 2 is a pictorial diagram of an alternative solar collector for use with the invention.

FIG. 3 is a schematic diagram of a heat storage body for use with the invention.

FIG. 4 is a schematic diagram of a heat transfer circuit for use with the invention.

FIG. 5 is a pictorial diagram of a combined heat storage facility and heat consuming engine mounted at the focus of a parabolic reflector.

FIG. 6 is a diagram showing the hot port of a Stirling engine placed on the heat storage medium of a heat storage facility and an array of heat conducting rods within the heat storage medium.

FIG. 7 is a variant of FIG. 6 showing a control interface between the hot port of the Stirling engine and the heat storage medium.

FIG. 8 is a flow diagram of the operation of the invention.

BEST MODES OF THE INVENTION

In a first example the system generates electrical energy from the sun using a Stirling engine. Referring first to FIG. 1, heat from the sun is collected and concentrated by an array of mirrors 10 having a focal point at the top of the tower 12 where the heat storage body 14 is mounted. The array of mirrors 10 are all mounted on heliostats that will turn to maintain a fixed focal point as the sun traverses the sky during the course of the day.

In an alternative arrangement FIG. 2 the light from the array of mirrors 10 is reflected from secondary reflector mounted at the top of tower 12, down onto heat storage body 14 standing on, or even below the ground.

The heat storage body 14 comprises a block of synthetic graphite 18 weighing ten tonnes, which is encased in thermal insulation 20; see FIG. 3. The block of graphite 18 comprises a cavity 22 to receive concentrated solar energy 8. There is also an aperture 24 on the underside of the insulation 20 to allow the concentrated solar energy to pass into the cavity in the heart of block 18. The concentrated solar energy enters the cavity 22 along the whole length of the aperture 24.

The energy received into the heat storage body 14 heats the block of graphite 18 raising its temperature. The block of graphite 18 can safely be heated to very high temperatures, for instance up to ˜2000° C. The heat energy stored in the graphite block 18 is then transferred at a controlled rate through a heat transfer circuit 30; see FIG. 4. Heat transfer circuit 30 delivers the heat to an electricity generator 40.

The graphite block 18 is made from many smaller graphite bricks assembled together. The bricks have passageways extending through them, indicated schematically at 32. Piping is laid into the passageways 32 to form a heat receiving part of the heat transfer circuit 30. Typically the piping will have high thermal conductivity, for instance by being fabricated from a high alloy of stainless steel, such as 253MA.

The heat transfer circuit 30 also includes piping 34 extending from the block to the electricity generator 40. In this part of the circuit it is useful for the piping to be well insulated and it is clad with thermally insulating material for this purpose. Similar piping 36 is used to return the heat transfer medium from the generator 40 back to the heat storage body 14. The heat transfer circuit is a closed system containing carbon dioxide (CO₂) working gas 39 that is continually recycled between the heat storage body 14 and the electricity generator 40.

In the return path 36 there is a regulator 38 for the heat transfer circuit 30. The purpose of the regulator 38 is to ensure that the working gas delivered to generator 40 is at the desired temperature and pressure; in this example 720° and 60 bar. Regulator 38 achieves temperature control by adjusting the rate of flow of the working gas through the graphite block 18.

In an alternative arrangement for the heat transfer medium piping, it is not embedded through the block but is laid in open channels that run along the surface of the block; so that the pipes are partly buried in the surface of the block.

In FIG. 4 the electricity generator 40 is shown to be a Stirling engine adapted with an external heat exchange element 52 at its hot port 54, and a sealed chamber 60 enclosing element 52 and the hot port 54. The sealed chamber 60 is in circuit with the heat transfer circuit 30, being connected between delivery pipe 34 and return pipe 36. At the Stirling engine the working gas is delivered to flood the sealed chamber 60 at the hot port 54 of the Stirling engine. Since this chamber contains the heat exchanger element 52 of the Stirling engine, this element is completely immersed in the delivered working gas.

Stirling engine 40 converts temperature difference to rotary motion by means of a flywheel 70. The Stirling engine has a cold port 56 as well as the hot port 54. In this case the cold port 56 has heat radiating fins 58 to shed heat. Hot gas received via the heat transfer circuit 30 heats element 52 that in turn heats the walls of the hot port 54 through an internal heat exchange element 62.

Very simply, the heat received at the hot port 54 of the Stirling engine causes the working gas 59 beneath the hot piston 64 to expand at constant pressure and so raise the power hot piston 66 causing flywheel 70 to rotate clockwise. Then, as hot piston 64 begins to fall the working gas 59 is moved to the cold port 56, where it is cooled at constant pressure and physically compressed; ready to be moved and heated again as the cycle continues.

The power stages are the two constant pressure stages, during which heat is either added or taken from the working gas, which causes its volume to increase or decrease, causing the tight fitting power piston 66 to move out or in, in order to maintain constant pressure.

Some of the power generated in these two stages is typically stored in flywheel 70, the momentum of which is used to physically expand or compress the gas in the other two stages. It also is used to move the gas from the hot port 54 of the engine to the cold port 56 and back past the loose fitting piston 64. The remainder of the power is available for doing work on an external load.

In this case this power is taken from the flywheel 70 to drive the coaxial rotor 80 of an alternating current (AC) electrical generator 82. The output voltage may be set at any convenient level, and it may be rectified if desired.

In an important alternative arrangement the heat storage body 14 and the electricity generator 40, which may be a Stirling engine, are connected to each other. In this case they may both be mounted at the focus of a reflector; such as a parabolic reflector dish 10 as shown in FIG. 5.

Whether the connected arrangement is at the focus of a reflector or not, a heat collecting element 60 of the heat consuming engine 40 is arranged in proximity, contact or embedded in the heat storage body 14 to supply heat energy to the heat consuming engine or process. In particular the heat collecting element may be an external heat exchanger element 52 at the hot port 54 of a Stirling engine 40 that collects heat from the heat storage facility by conduction, convection, radiation, or a combination of these three modes of heat transfer.

There are two alternatives under this ‘direct contact’ concept. The first of these is the ‘non-regulated’ alternative. In this case, the engine is basically permanently fixed to the graphite block. In this case there is no regulation of the heat input to the engine and therefore the engine will run continuously until the energy in the block runs down so far that the internal friction forces in the Stirling engine cause it to stop. An example of this arrangement is shown in FIG. 6 where the heat storage body 14 comprises a block of graphite 18 surrounded by a heat insulating jacket 20. An aperture in the insulation 26 in the top of body 14 receives the hot port 54 of a Stirling engine 40.

In the arrangement of FIG. 6 the top of the graphite block 18 is flat, and the lower surface of the hot port 54 of the Stirling engine 40 is also flat. Together they form a junction through which heat is transferred from the graphite into the Stirling engine, which converts the heat energy to electrical energy. Of course these surfaces need not be flat and they need not match each other.

A form of simple passive regulation of heat transfer may involve inserting a number of solid rods 70 made from a material with high thermal conductivity relative to the graphite into the graphite to act as ‘heat highways’ and help to distribute the heat throughout the block more effectively. The concept is shown in principle in FIG. 6.

The second alternative is a ‘regulated’ version of the ‘direct contact’ concept. This involves regulating the amount of heat entering the Stirling engine 40, potentially by regulating the thermal resistance between the engine 40 and the graphite block 18. Regulation could, for instance, be achieved by a number of means, including but not limited to;

-   -   Varying the properties of a control interface 28 (see FIG. 7)         between the Stirling engine 40 and the block 18; or     -   Creating an intermittent separation between the engine 40 and         block 18 (or interface 28).

The control interface 28 of FIG. 7 sits directly on the surface of the graphite block 18 exposed by the aperture 26 in the insulation 20. The control interface 28 covers the entire area of the exposed surface. On top of the control interface 28 is the hot port 54 of the Stirling engine 40.

Regulation of heat transfer through the control interface 28 allows the engine to be effectively turned On and Off, or regulated in some more linear fashion. It may also allow the regulation of their electrical output. Such control facilitates modularity since there are little or no shared items of plant and no interlinking piping.

An active control interface 28 could involve the use of a heat transfer medium between the surfaces of the graphite block 18 and hot port 54 through which heat passes. In this case there could be a cavity at the point of contact which is flooded with heat exchange medium. Then, heat transfer may be regulated, for instance by controlling the pressure of the heat transfer medium. Other possibilities include controlling the flow rate of the heat transfer medium through a heat exchange circuit.

An intermittent separation might be accomplished making use of the differential thermal expansion provided by different materials (like the bi-metallic strip). A mechanism could be constructed using this principle and arranged to respond to the temperature at the hot port of the Stirling engine to move the engine relative to the block. A change in the distance between them may be used to regulate the heat transfer and keep the temperature of hot port of the Stirling engine within its preferred operational range.

Another possibility is that the contact surfaces at the junction could be shaped, for instance helically, so that the degree of contact between them changes as the engine is rotated relative to the block.

The mechanism could be applied around the hot port of the Stirling engine, or between the hot port and the block. An air gap may be created when the mechanism responds to excess heat, providing a binary step function control. Alternatively, some material may be used to fill the space between the engine and block, for instance a fluid filled bladder or piston may occupy the space and move as the gap changes to provide smoother control.

Referring now to FIG. 8, the overall operational cycle 90 of this example will be described with reference to the flowchart.

First solar energy is collected 92 by the mirror array 10.

Next energy collected and concentrated by the mirrors is stored 94 in the graphite block 18 as heat.

Next a surface of the graphite block 18 and a surface of a hot port 54 of a heat consuming engine or process are arranged 96, such that a surface of the hot port is in proximity to, in contact with graphite block 18 or embedded in a surface of the heat storage medium.

Then, transferring stored heat energy 98 from the surface of the graphite block to the surface of the hot port.

The heat transfer process is controlled 100.

Finally, the heat received at the Stirling engine drives a generator to produce electricity 102.

As an example for an implementation, the following technical parameters are provided:

Rated Electrical Generation 30 kW Stirling operating temperature 725° C. Graphite block operational temperature range 720-1,200° C. Mirror surface area 1,100 m² Graphite block weight 10 tonnes

With a thermal to electrical efficiency of 30% and a block efficiency of 95% such a system could run for twenty four hours after heating the graphite block to 1,200° C.

Although the invention has been described with reference to a particular example, it will be appreciated by the appropriately skilled person that many modifications and additions are possible. For instance, other working gases besides carbon dioxide could be used, as could other mediums. Also, the heat storage facility need not be located at the top of a tower, but could be on the ground, or even under the ground in a geothermal application. The heat storage facility could be any convenient size. 

1. A method for releasing stored heat energy to do useful work, comprising: storing heat energy in a heat storage medium; arranging a surface of the heat storage medium and a surface of a heat collecting element of a heat consuming engine or process such that a surface of the heat collecting element is in proximity to, in contact with, or embedded in a surface of the heat storage medium such that heat energy is able to be transferred directly from the surface of the heat storage medium to the surface of the heat collecting element; then, transferring stored heat energy from the surface of the heat storage medium to the surface of the heat collecting element.
 2. A method according to claim 1, wherein the surface of the medium through which heat passes is a flat part of the outer surface of the medium, similarly, the surface of the element through which heat passes is a flat part of the outer surface of the element, and the two flat parts, of the medium and element, are arranged in contact with each other to form a junction.
 3. A method according to claim 2 wherein the rate of heat flow from the surface of the heat storage medium into the heat collecting element is controlled by altering the position, size or shape of the surface of the medium or element, or both.
 4. A method according to claim 1, wherein the heat storage medium is equipped with heat transfer pathways that change the thermal gradients within the medium to regulate the flow of heat out of the surface and into the element.
 5. A method according to claim 4, where heat transfer pathways are attached to the surface, or inserted through the heat storage medium.
 6. A method according to claim 1, where there is a control interface interposed between the heat collecting element and the heat storage medium to control the flow of heat between the surfaces of the medium and element through which heat passes.
 7. A method according to claim 6, where a passive control interface dissipates excessive heat when it is above a given temperature, or recovers heat from other parts of the heat storage medium when the temperature falls, or both.
 8. A method according to claim 6, where an active control interface effects control by reconfiguring itself, or by adjusting the size, shape or location of the area it has in contact with the surface of the medium, or element, or both.
 9. A method according to claim 8, where the distance between the surfaces of the heat collecting element and the heat storage medium through which heat passes is changed.
 10. A method according to claim 1, where the heat consuming engine is a Stirling engine, and the hot port of a Stirling engine is brought into contact with a surface of the heat storage medium.
 11. Apparatus for releasing stored heat energy to do useful work, comprising: a heat storage medium having a heat transfer surface from which heat is transferred from the heat storage medium; a heat consuming engine or process having a heat collecting element which also has a heat transfer surface to receive heat; wherein, the heat transfer surface of the heat storage medium and the heat transfer surface of the heat collecting element are arranged such that the surface of the heat collecting element is in proximity to, in contact with, or embedded in the surface of the heat storage medium such that heat energy is able to be transferred directly from the surface of the heat storage medium to the surface of the heat collecting element, and to allow transfer of stored heat energy from the heat storage medium to the heat collecting element.
 12. Apparatus according to claim 26, further comprising: a closed loop heat transfer circuit containing a recirculating heat transfer medium, having a heat receiving element extending through the heat storage body and a heat delivery element to supply heat energy; and a regulator for the closed loop heat transfer circuit to govern the temperature of the supplied heat independently of the temperature of the heat storage facility. 13.-16. (canceled)
 17. A method according to claim 8, where the active control interface incorporates differential thermal expansion.
 18. A method according to claim 8, where the active control interface has shaped surfaces that allow the degree of contact between the heat collecting element and the heat storage medium to be varied by rotation of one relative to the other.
 19. A method according to claim 8, where the active control interface comprises a fluid heat transfer medium and heat transfer is regulated by controlling the pressure of the heat transfer medium.
 20. Apparatus according to claim 11, wherein the surface of the medium through which heat passes is a flat part of the outer surface of the medium, similarly, the surface of the element through which heat passes is a flat part of the outer surface of the element, and the two flat parts, of the medium and element, are arranged in contact with each other to form a junction.
 21. Apparatus according to claim 20 wherein the rate of heat flow from the surface of the heat storage medium into the heat collecting element is controlled by altering the position, size or shape of the surface of the medium or element, or both.
 22. Apparatus according to claim 11, wherein the heat storage medium is equipped with heat transfer pathways that change the thermal gradients within the medium to regulate the flow of heat out of the surface and into the element.
 23. Apparatus according to claim 22, where heat transfer pathways are attached to the surface, or inserted through the heat storage medium.
 24. Apparatus according to claim 11, further comprising a control interface interposed between the heat collecting element and the heat storage medium to control the flow of heat between the surfaces of the medium and element through which heat passes.
 25. Apparatus according to claim 24, further comprising a passive control interface that dissipates excessive heat when it is above a given temperature, or recovers heat from other parts of the heat storage medium when the temperature falls, or both.
 26. Apparatus according to claim 24, where an active control interface effects control by reconfiguring itself, or by adjusting the size, shape or location of the area it has in contact with the surface of the medium, or element, or both.
 27. Apparatus according to claim 26, where the active control interfaces changes the distance between the surfaces of the heat collecting element and the heat storage medium through which heat passes is changed.
 28. Apparatus according to claim 26, where the active control interface incorporates differential thermal expansion.
 29. Apparatus according to claim 26, where the active control interface has shaped surfaces that allow the degree of contact between the heat collecting element and the heat storage medium to be varied by rotation of one relative to the other.
 30. Apparatus according to claim 26, where the active control interface comprises a fluid heat transfer medium and heat transfer is regulated by controlling the pressure of the heat transfer medium.
 31. Apparatus according to claim 11, where the heat consuming engine is a Stirling engine, and the hot port of a Stirling engine is brought into contact with a surface of the heat storage medium. 